It is well known that polymers having sulfonic acid group are useful for a polymer electrolyte to be applied to polymer electrolyte membrane fuel cells and the like. The following polymers, for example, are proposed as a polymer electrolyte to be applied to polymer electrolyte membrane fuel cells and the like: polymers introducing sulfonic acid group into fluorine-containing polymers, typically being Nafion (a trade name of DuPont Co., Ltd.); polymers introducing sulfonic acid group into poly(ether ketone)s (U.S. Pat. No. 5,438,082); polymers introducing sulfonic acid group into poly(ether sulfone)s (J. Membrane Science, 83, 211 (1993)); polymers introducing sulfonic acid group into polyimides (Kokai (Japan unexamined patent publication) No. 2003-277501); polymers introducing sulfonic acid group into polyphenylenes (U.S. Pat. No. 5,403,675); and polymers introducing sulfonic acid group into polyphosphazenes (Chemical Material, 3, 1120, (1991)).
On the other hand, oxocarbons, for example squaric acid, croconic acid and the like, are known as high acidic functional groups due to a stable resonated structure dissociating hydrogen atom from the oxocarbon group (Oxocarbons, page 45 (Edited by Robert West), Academic Press (1980), (ISBN:0-12-744580-3); Journal of the American Chemical Society, 95, 8703 (1973)). Recently polymers having oxocarbon groups instead of sulfonic acid groups are proposed as a polymer electrolyte (Kokai (Japan unexamined patent publication) No. 2006-225624).